Credits are typically applied to invoices through a zero$ cash receipt, but that does not happen automatically.

Customer Maintenance, Invoices tab, lower grid... shows details of transactions against open AR balances for each open invoice.

Thank you. I was just having this conversation that there are too many hands able to get into AR and I don't know who applied these credits. I can see on the invoices tab the transactions I just don't know who did it.

Sage 100 does leave an audit trail for all transactions, but it sometimes isn't easy to find certain details.  Unfortunately that grid doesn't allow you to link in the "who".

If you use Paperless Office, find the journal and you can see the User Logon in the list (even if you don't have the option enabled to print the user in the PDF footer).

I'd query AR_TransactionPaymentHistory, linked to SY_User (translating UserUpdatedKey into a name).

Unfortunately I don't see that paperless has been set up for Journal and Registers.

I believe that the Payment Reference of CREDIT-X happens when you use the AR Utility to "Match Credits to Open Invoices".  

https://help-sage100.na.sage.com/2018/Subsystems/AR/ARPEND/MATCH_CREDITS_TO_OPEN_INV_OVR.htm 

YES! This is exactly what happened. I tested it myself in a test company. I was also able to find out who did it by searching the activity log in the Library Master.